Digital Subscriber Line (DSL) is a modem technology that enables broadband digital data to be transmitted over twisted-pair wire, which is the type of infrastructure that links most home and small business subscribers to their telephone service providers. DSL modems enable users to access digital networks at speeds tens to hundreds of times faster than current analog modems and basic ISDN service. A range of DSL standards have been defined, known generically as “xDSL,” wherein the various standards have different data rates and other associated features but share common principles of operation.
The present patent application is concerned mainly with Asymmetric DSL (ADSL) service, which allows data to be conveyed downstream from a central office to subscribers at a rate up to about 8 Mbit/s, and to transport an upstream signal from the subscribers at a rate of about 640 kbit/s. ADSL is rate-adaptive, and the actual rate of data transmission over each subscriber line is determined according to noise conditions and the length of the line. Consequently, subscribers located far from the central office may be limited to low bit rates, and some distant subscribers cannot be offered even minimal service. There is therefore a need to improve the performance of ADSL modems at low rates and high reaches, in order to increase the coverage that can be offered by a given central office.
ADSL is based on a discrete multi-tone (DMT) transmission system, in which data are encoded using 256 different “tones,” each corresponding to a frequency band 4.3125 kHz wide. Each DMT symbol is a mapping of a fixed number of bits to sine waves of multiple frequencies. The number of bits per symbol is determined according to the line quality, and is typically low for long and noisy lines.
Prior to data encoding in a DMT modem for ADSL transmission, the input data are assembled into groups, or frames. Conventional ADSL framing is defined in Recommendation G.992.1 (ex G.dmt) of the International Telecommunication Union (ITU), which is incorporated herein by reference. A predetermined number (S) of framing bytes (8 bits each) is added to each frame of payload data. A Reed-Solomon encoder adds P parity bytes (also known as check bytes, error correction bytes or redundancy bytes) to each frame, P an integer multiple of S, in order to produce a Reed-Solomon codeword, or output frame, of N bytes. In the context of the present patent application and in the claims, the framing bytes and parity bytes are collectively referred to as “overhead bytes.” The codeword is then divided into Q equal parts of B bytes each. Each such part is mapped to a DMT symbol.
For long lines, B is small, and therefore the percentage of the overhead is large. The high overhead stems from the fixed amount of overhead per symbol that is required by the ADSL standard. There is typically one framing byte per symbol (i.e., the number of framing bytes S is equal to the number of symbols Q), and at least one parity byte. When there are relatively few bytes per symbol, much or most of the available bandwidth may be taken up by the overhead. This substantial constant overhead engendered by ADSL standards limits the possibility of providing ADSL service over long lines.
Methods have been proposed for improving ADSL framing by reduction of the number of overhead bytes per codeword. For example, ITU temporary document MA-027, which is incorporated herein by reference, proposes a protocol for on-line rate adaptation using constant percentage overhead framing. This document was published on Mar. 29, 1999, after the date of conception of the present invention. According to the protocol proposed in MA-027, DMT frames (or symbols) are decoupled from input data frames and Reed-Solomon codewords, so that the DMT frames are not aligned with or the same size as the data frames and codewords. Thus, a DMT frame will not necessarily contain a fixed number of overhead bytes as in standard ADSL framing. The document presents one example in which each DMT frame includes 0.35 bytes of overhead (check bytes plus framing bytes).